1. Field of the Invention
The present invention relates to electronics. More specifically, the present invention relates to output drivers for digital circuits.
2. Description of the Related Art
Electronic circuits typically include several output drivers used to pass signals from one chip to another, or from one circuit board to another. An example of this is an M-bit analog to digital converter (ADC) driving an external circuit. In this case, a differential output driver is required for each bit (for a total of M differential output drivers), as well as additional output drivers for the saturation and “data valid” bits. Another example is a multi-channel deserializer or switch matrix where the number of differential output drivers could be greater than 100.
The objective of a digital output driver is to swing the output voltage from one defined level to another as rapidly as possible. Due to the capacitive load on the output driver, in order to make a fast transition between logic states, the output driver must sink or source a large current into the output transmission line in order to provide an output signal having logic high and logic low signals within their specified voltage ranges. The rising transition time (to go from logic low to logic high) of a typical output driver is relatively short. The falling transition time (to go from logic high to logic low), however, is usually much longer because the rate at which the load capacitance can discharge is set by the current sink. The larger the current sink is, the faster the trailing edge will go down. This is, of course, a tradeoff with power dissipation. The larger the current sink is, the more power the circuit will dissipate.
Digital output drivers must therefore provide adequate current sourcing and sinking capability in order to drive capacitive and resistive loads at the required speed. If differential outputs are required, then two current sources are needed for each driver. For circuits having several output drivers, considerable power is dissipated by the need to have several current sources providing drive and sink currents for the output drivers.
Hence, there is a need in the art for an improved output driver offering lower power consumption than prior art output drivers, which is operable at high speeds.